Discharge of an ac capacitor using totem-pole power factor correction (pfc) circuitry

ABSTRACT

An AC capacitor is coupled to a totem-pole type PFC circuit. In response to detection of a power input disconnection, the PFC circuit is controlled to discharge the AC capacitor. The PFC circuit includes a resistor and a first MOSFET and a second MOSFET coupled in series between DC output nodes with a common node coupled to the AC capacitor. When the disconnection event is detected, one of the first and second MOSFETs is turned on to discharge the AC capacitor with a current flowing through the resistor and the turned on MOSFET. Furthermore, a thyristor may be simultaneously turned on, with the discharge current flowing through a series coupling of the MOSFET, resistor and thyristor. Disconnection is detected by detecting a zero-crossing failure of an AC power input voltage or lack of input voltage decrease or input current increase in response to MOSFET turn on for a DC input.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of United States Application for patentSer. No. 16/858,907 filed Apr. 27, 2020, the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

The present disclosure generally concerns electronic circuits and moreparticularly circuits configured to be coupled to an AC voltage source,such as the electric power distribution mains. The present disclosuremore particularly applies to circuits comprising an AC capacitor.

BACKGROUND

In many applications, the power received from the electric powerdistribution mains by devices connected thereto is filtered by acapacitor (such as an AC capacitor) upstream of a voltage conversionand/or power factor correction circuit. The capacitor, for example, isgenerally directly connected to the line and neutral conductors (orbetween two phases) of the AC power supply, or alternatively directlyconnected to lines of a DC power supply.

The presence of the capacitor requires discharging it when the device isdisconnected from the power supply network by the user. Indeed, thecharge that is stored by the capacitor at the time when the device isdisconnected can be quite high and presents a significant danger to theuser.

Although many solutions exist for this problem, they are often complexand/or expensive. There is a need in the art for a simple andinexpensive AC capacitor discharge circuit.

SUMMARY

In an embodiment, a circuit comprises: a first capacitor having firstand second electrodes respectively coupled to first and second powersupply input nodes; a first switching transistor having a conductionpath coupled between the first power supply input node and a first DCnode; a second switching transistor having a conduction path coupledbetween the first power supply input node and a second DC node; a firstthyristor having a conduction path coupled between the second powersupply input node and the first DC node; a second thyristor having aconduction path coupled between the second power supply input node andthe second DC node; a resistor coupled between the second power supplyinput node and an intermediate node; a first diode having a conductionpath coupled between the intermediate node and the first DC node; asecond diode having a conduction path coupled between the intermediatenode and the second DC node; and a control circuit configured to sense adisconnection of input power to the first and second power supply inputnodes and in response thereto turn on one of the first and secondswitching transistors to discharge the first capacitor through theresistor.

In an embodiment, a circuit comprises: a first capacitor having firstand second electrodes respectively coupled to first and second powersupply input nodes; a first switching transistor having a conductionpath coupled between the first power supply input node and a first DCnode; a second switching transistor having a conduction path coupledbetween the first power supply input node and a second DC node; a firstthyristor having a conduction path coupled between the second powersupply input node and the first DC node; a second thyristor having aconduction path coupled between the second power supply input node andthe second DC node; a resistor coupled between the second power supplyinput node and an intermediate node; a third thyristor having aconduction path coupled between the intermediate node and the first DCnode; a fourth thyristor having a conduction path coupled between theintermediate node and the second DC node; and a control circuitconfigured to sense a disconnection of input power to the first andsecond power supply input nodes and in response thereto turn on one ofthe first and second switching transistors and one of the third andfourth thyristors to discharge the AC capacitor through the resistor.

In an embodiment, a circuit comprises: a first capacitor having firstand second electrodes respectively coupled to first and second powersupply input nodes; a first switching transistor having a conductionpath coupled between the first power supply input node and a first DCnode; a second switching transistor having a conduction path coupledbetween the first power supply input node and a second DC node; a firstthyristor having a conduction path coupled between the second powersupply input node and the first DC node; a second thyristor having aconduction path coupled between the second power supply input node andthe second DC node; a first resistor and a third thyristor having aconduction path coupled in series between the second power supply inputnode and the first DC node; a second resistor and a fourth thyristorhaving a conduction path coupled in series between the second powersupply input node and the second DC node; and a control circuitconfigured to sense a disconnection of an AC input to the first andsecond AC nodes and in response thereto turn on one of the first andsecond switching transistors and one of the third and fourth thyristorsto discharge the AC capacitor through one of the first and secondresistors coupled in series with said one of the third and fourththyristors.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages will be discussed indetail in the following non-limiting description of specific embodimentsin connection with the accompanying drawings, in which:

FIG. 1 shows an example of a power conversion system;

FIG. 2 schematically shows an embodiment of a power conversion system orcircuit equipped with an AC capacitor discharge function;

FIG. 3 shows waveforms for the operation of the PFC rectifier circuit innormal mode;

FIGS. 4A and 4B show waveforms for the operation of the PFC rectifiercircuit in detecting and responding to a disconnection of an AC inputvoltage by discharging an AC capacitor;

FIG. 5 schematically shows an embodiment of a power conversion system orcircuit equipped with an AC capacitor discharge function;

FIGS. 6A and 6B show waveforms for the operation of the PFC rectifiercircuit in detecting and responding to a disconnection of an AC inputvoltage by discharging an AC capacitor;

FIG. 7 shows an alternate thyristor gating configuration for the circuitof FIG. 2;

FIG. 8 shows an alternate thyristor gating configuration for the circuitof FIG. 5 (as well as for the circuit of FIGS. 9); and

FIG. 9 schematically shows an embodiment of a power conversion system orcircuit equipped with an AC capacitor discharge function.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the different drawings. In particular, the structural and/orfunctional elements common to the different embodiments may bedesignated with the same reference numerals and may have identicalstructural, dimensional, and material properties.

For clarity, only those steps and elements which are useful to theunderstanding of the described embodiments have been shown and aredetailed. In particular, the DC/AC or DC/DC power converter powered bythe described circuit as well as the control of such a power converterhave not been detailed, the described embodiments being compatible withusual converters and usual controls of such converters.

Throughout the present disclosure, the term “connected” is used todesignate a direct electrical connection between circuit elements withno intermediate elements other than conductors, whereas the term“coupled” is used to designate an electrical connection between circuitelements that may be direct, or may be via one or more intermediateelements.

The terms “about”, “substantially”, and “approximately” are used hereinto designate a tolerance of plus or minus 10%, preferably of plus orminus 5%, of the value in question.

FIG. 1 shows an example of a power conversion system 1. Such aconversion system 1 is based on a halfwave or fullwave rectification andpower factor correction (PFC) of an AC power supply voltage Vac,followed by a DC/DC or DC/AC conversion to power a load.

Schematically, AC voltage Vac is applied between two input terminals 11(L—line) and 13 (N—neutral) coupled to AC input terminals 21 and 23 of apower factor correction (PFC) rectifier circuit 3. Voltage Vac is, forexample, the AC voltage or mains voltage of a 230V/50 Hz or 60 Hz, or110V/50 Hz or 60 Hz power distribution network. Typically, terminals 11and 13 are formed of pins of a plug of connection of system 1 to asocket of an electrical installation.

Output terminals 25 and 27 of the PFC rectifier circuit 3 are coupled toinput terminals 51 and 53 of a DC/DC or DC/AC conversion circuit 5.Output terminals 55 and 57 (DC or AC according to the embodiment) ofcircuit 5 provide a power supply voltage to a load 7. A DC capacitor Cdccouples, preferably connects, terminals 25 and 27 to smooth therectified voltage and deliver a rectified voltage at the input ofcircuit 5. Converter 5 is preferably a switched-mode power supplycontrolled at a frequency much higher (by a ratio in the order of from1,000 to 10,000) than the frequency of voltage Vac.

In the applications targeted by the present disclosure, a capacitor(referred to as an AC capacitor) Xcap couples, preferably connects,power supply terminals 11 and 13 upstream of any element of conversionsystem 1 and in particular upstream of the PFC rectifier circuit 3. Thefunction of capacitor Xcap is to filter AC voltage Vac, particularly toremove possible high-frequency disturbances (at a frequency greater thanthe frequency of AC voltage Vac).

The presence of the AC capacitor Xcap at the power supply input requiresdischarging this capacitor for user safety reasons when the system isdisconnected from the electrical installation (i.e., the power mains, ACor DC) in case of a contact being made by the user with terminals 11 and13 when the system is disconnected.

Many solutions to discharge the AC capacitor in response todisconnection from the AC input voltage are known in the art.

A first category of solutions uses passive components, with capacitorXcap then forming part of an AC filter having a resistor of low valueconnected in parallel with the capacitor and dissipating the power thatit contains when the system is disconnected. A disadvantage of such asolution is that permanent power dissipation exists in the application.

A second category of solutions uses active components to control adischarge of the AC capacitor when power supply voltage Vac disappears.Such solutions generally require additional circuits and componentswhich increase the cost of the system or of the application.

According to the described embodiments herein, it is provided to takeadvantage of a specific circuit structure present within the PFCrectifier circuit 3 to support the capacitor discharge operation.

Reference is now made to FIG. 2 which schematically shows an embodimentof a power conversion system or circuit 101 equipped with an ACcapacitor discharge function. An AC power supply voltage Vac is appliedto input terminals 100 and 102. A capacitor (for example, the AC (input)capacitor) Xcap is coupled, preferably connected, to and between inputterminals 100 and 102, perhaps as part of a filter circuit 103 whichalso includes an inductor 104 and a capacitor 106 coupled in seriesbetween terminal 100 and a ground node 108, and an inductor 110 and acapacitor 112 coupled in series between terminal 102 and the ground node108. A PFC rectifier circuit 120 of the totem-pole type includes a firsttransistor switch 122 (for example, of the n-channel MOSFET type) havinga source-drain path coupled between an input node 124 (at the seriesconnection of inductor 104 and capacitor 106) and a first rectifiedoutput node 126. The PFC rectifier circuit 120 further includes a secondtransistor switch 132 (for example, of the n-channel MOSFET type) havinga source-drain path coupled between the input node 124 and a secondrectified output node 136. Each of the transistors 122 and 132 includesan intrinsic body diode as shown. An external diode could be added insome cases in reverse parallel of each of the transistors 122 and 132.The PFC rectifier circuit 120 further includes a first thyristor 140(for example, of the cathode-gated type) having a conduction pathcoupled between the input node 134 (at the series connection of inductor110 and capacitor 112) and the first rectified output node 126, and asecond thyristor 142 (for example, of the cathode-gated type) having aconduction path coupled between the input node 134 and the secondrectified output node 136. A differential mode inductor 128 has a firstterminal connected to node 124 and a second terminal connected to node130 at the series connection of transistors 122 and 132. A surgeprotection diode may be provided between nodes 124 and 126 and a surgeprotection diode may be provided between nodes 134 and 136. An in-rushcurrent limiting resistor Rid (for example, of the negative temperaturecurrent (NTC) type) is connected to and between the input node 134 atthe series connection of thyristors 140 and 142 and an intermediate node148. The PFC rectifier circuit 120 further includes a first diode 150having a conduction path coupled between the intermediate node 148 andthe first rectified output node 126, and a second diode 152 having aconduction path coupled between the intermediate node 148 and the secondrectified output node 136. A DC capacitor Cdc couples, preferablyconnects, rectified output nodes 126 and 136 to smooth the rectifiedvoltage and deliver a rectified voltage to downstream circuits (forexample, at lines 51, 53 to a converter circuit 5 as shown in FIG. 1).The control signals M1 and M2 for driving the gates of the first andsecond transistor switches 122 and 132, respectively, are generated by acontrol circuit 160. The control circuit 160 further generates thecontrol signals G1 and G2 which drive the cathode gates of the first andsecond thyristors 140 and 142, respectively. The control circuit 160includes inputs for sensing the voltage at input nodes 124 and 134, andfurther includes an input for sensing AC current flow using a currentsensor 162 coupled to sense current flowing from or to the power supply(for example, AC input) terminals 100, 102.

Although the first and second thyristors 140 and 142 are shown in FIG. 2as both being cathode-gates devices, the second thyristor 142 couldinstead be an anode-gated device as shown in FIG. 7. Furthermore, theMOSFET devices used for the switching transistors can be implemented inany suitable technology (including, without limitation, the use ofsilicon or high-gap materials such as SiC or GaN).

FIG. 3 shows waveforms for the operation of the PFC rectifier circuit120. Waveform 170 is the voltage across the AC capacitor Xcap that issensed by the control circuit 160 at input nodes 124 and 134. In anormal operating mode, the sinusoidal shape of waveform 170 isindicative of the receipt of the AC power supply voltage Vac as appliedto input terminals 100 and 102.

During a positive phase of the AC power supply voltage Vac, the controlcircuit 160 generates the signal G2 (reference 172) to cause the secondthyristor 142 to turn on (with the first thyristor 140 controlled to beturned off) and generates the signal M2 with pulses at a high frequency(reference 174, for example, with a pulse width modulation (PWM)), tocontrol turning on/off of the second transistor switch 132 (with thefirst transistor switch 122 turned off). The signal G2 is generated bythe control circuit 160 for a duration of time sufficient to ensure thatthe second thyristor 142 is turned on for substantially the entireduration of the positive phase of the AC power supply voltage Vac (andfor at least as long as the conduction period (duration 176) of thepulses for signal M2). In an alternative implementation, the signal G2need only be a pulse of sufficient duration to cause the secondthyristor 142 to turn on in forward conduction mode up to its currentexceeding the latching current value, then the second thyristor 142 willremain in the on state until the current passing through the conductionpath falls below the holding current value of the device.

With respect to the positive phase of the AC power supply voltage Vac,when second transistor switch 132 is turned on in response to the pulsedsignal M2 generated by the control circuit 160, inductor current flowsfrom node 124 through turned on transistor switch M2, then throughturned on second thyristor 142 to node 134. When second transistorswitch 132 is turned off in response to the pulsed signal M2, inductorcurrent flows from node 124 through the freewheeling body diode oftransistor switch 122 (which is turned off by signal M1) to chargecapacitor Cdc and return through turned on second thyristor 142 to node134. Signal M1 could also be activated during this period (after adead-time to avoid transistors 122 and 132 to be conducting in forwarddirection at the same time) to ensure the reverse conduction of thetransistor 122 channel and reduce its conduction losses.

During a negative phase of the AC power supply voltage Vac, the controlcircuit 160 generates the signal G1 (reference 178) to cause the firstthyristor 140 to turn on (with the second thyristor 142 controlled to beturned off) and generates the signal M1 with a pulsed signal at a highfrequency (reference 180, for example, with a pulse width modulation(PWM)), to control turning on/off of the first transistor switch 122(with the second transistor switch 132 turned off). The signal G1 isgenerated by the control circuit 160 to ensure that the first thyristor140 is turned on for substantially the entire duration of the negativephase of the AC power supply voltage Vac (and for at least as long asthe conduction period (duration 182) of the pulses for signal M1). In analternative implementation, the signal G1 need only be a pulse ofsufficient duration to cause the first thyristor 140 to turn on inforward conduction mode up to its current exceeding the latching currentvalue, then the first thyristor 140 will remain in the on state untilthe current passing through the conduction path falls below the holdingcurrent value of the device. p With respect to the negative phase of theAC power supply voltage Vac, when the first transistor switch 122 isturned on in response to the pulsed signal M1 generated by the controlcircuit 160, inductor current flows from node 134 through turned onfirst thyristor 140, then through turned on transistor switch M1 to node124. When first transistor switch 122 is turned off in response to thepulsed signal M2, inductor current flows from node 134 through turned onfirst thyristor 140 to charge capacitor Cdc and return through thefreewheeling body diode of transistor switch 132 (which is turned off bysignal M2) to node 124. Signal M2 could also be activated during thisperiod (after a dead-time to avoid transistors 122 and 132 to beconducting in forward direction at the same time) to ensure the reverseconduction of the transistor 132 channel and reduce its conductionlosses.

The current sensed by current sensor 162 is used by the control circuit160 to control the conduction periods 182 and 176 of the first andsecond transistor switches 122 and 132, respectively.

The pulse frequency of the signals M1 and M2 is generally fixed by a PWMclock and is typically higher by a factor of at least 100 than thefrequency of the AC power supply voltage Vac.

Consider now the operating scenario where the circuit 101 isdisconnected from the input power supply (for example, from AC powersupply voltage Vac) applied to input terminals 100 and 102. The chargeon the AC capacitor Xcap needs to be discharged and the circuit 101includes an AC capacitor discharge function using the circuitry of thePFC circuit itself to accomplish this goal. The control circuit 160 cansense the disconnection of the circuit 101 from the power supply (forexample, the AC power supply voltage Vac) by monitoring the voltage atthe input nodes 124 and 134. If that sensed voltage at input nodes 124and 134 fails to zero-cross (i.e., the voltage difference between inputnodes 124 and 134 does not fall to zero), this is indicative of theoccurrence of a disconnection of the AC power supply voltage Vac. Inresponse to sensing the disconnection event, the control circuit willselectively turn on one of the transistor switches 122 or 132 of the PFCcircuit, depending on polarity of the voltage across AC capacitor Xcap,in order to discharge the stored charge. The control circuit 160 willcontinue to monitor the voltage difference between input nodes 124 and134 and when that difference falls to zero the previously turned ontransistor switch 122 or 132 will be turned off.

In an alternative embodiment, at first plug-in of the converter, thecontrol circuit may sense the rising edge (or falling edge) of thesupply voltage. Then, in the case where the voltage between input nodes124 and 134 does not fall to zero and in the case if after one of thetransistor 122 and 132 is turned on and that the current sensor sensesenough current increase and/or that the voltage between input nodes 124and 134 does not collapse (after transistors 122 and 132 are turned on),the control circuit may interpret this information as the converterhaving been plugged in to a DC voltage network to receive a DC supplyvoltage). The Xcap discharge function will then be ensured (in caseoperation under DC voltage network is assumed) once the inductor currentdoes not increase (to a certain degree, for example, anymore) or, incase where the PFC circuit is not activated (for example delayed)repetitively at low-frequency (for example 1 Hz or lower) to ensuresafety while not dissipating too much energy. Or, the above sequence(which is activated at first plug-in) will be repeated at the samefrequency to detect whether the DC supply voltage is still present.

For example, in the event of a disconnection (FIG. 4A, reference 190,time td) where the polarity of the voltage across AC capacitor Xcap ispositive (i.e., powered off under a positive halfwave with theconventions taken in the drawings), the control circuit 160 will detectthrough monitoring of the voltage at input nodes 124 and 134 that nozero-cross occurs (FIG. 4A, reference 192; or alternatively that thereis a DC supply disconnection as discussed above) within a certain timeperiod (FIG. 4A, reference 194, from time td to time t1), wherein thecertain time period may, for example, be in the range of one AC cycle upto about 1-3 seconds. In response to detection of the disconnectionevent, the control circuit 160 asserts the signal M2 (FIG. 4A, reference196, at time t1) to turn on the second transistor switch 132. Thevoltage across the AC capacitor Xcap is discharged (FIG. 4A, reference198) by a capacitor discharge current that flows from node 100 at thepositive terminal of AC capacitor Xcap through inductor 104 to inputnode 124, then through turned on second transistor switch 132 to forwardbias and flow through second diode 152 to node 148, then throughresistor Rid to node 134, and then through inductor 110 to node 102 atthe negative terminal of AC capacitor Xcap. The control circuit 160turns on the second transistor switch 132 through assertion of signal M2for a length of time (FIG. 4A, reference 200, time t1 to time t2) thatis sufficient to ensure full discharge of the AC capacitor Xcap. Thetime period 200 will, for example, extend for a duration equal to manycycles of the PWM clock and, in particular, may be equal to aboutone-quarter of the period up to about few periods of the AC power supplyvoltage Vac. When the control circuit 160 detects that the voltagedifference between input nodes 124 and 134 falls to zero, the signal M2is preferably deasserted to turn off transistor switch 132.

Conversely, in the event of a disconnection (FIG. 4B, reference 190,time td) where the polarity of the voltage across AC capacitor Xcap isnegative (i.e., powered off under a negative halfwave with theconventions taken in the drawings), the control circuit 160 will detectthrough monitoring of the voltage at input nodes 124 and 134 that nozero-cross occurs (FIG. 4B, reference 192; or alternatively that thereis a DC supply disconnection as discussed above) within a certain timeperiod (FIG. 4B, reference 194, from time td to time t1), wherein thecertain time period may, for example, be in the range of one AC cycle upto about 1-3 seconds. In response thereto, the control circuit 160asserts the signal M1 (FIG. 4B, reference 196, at time t1) to turn onthe first transistor switch 122. The voltage across the AC capacitorXcap is discharged (FIG. 4B, reference 198) by a capacitor dischargecurrent that flows from node 102 at the negative terminal of ACcapacitor Xcap through inductor 110 to input node 134, then throughresistor Ricl to node 148, to forward bias and flow through first diode150, then through turned on first transistor switch 122 to node 124, andthen through inductor 104 to node 100 at the positive terminal of ACcapacitor Xcap. The control circuit 160 turns on the first transistorswitch 122 through assertion of signal M1 for a length of time (FIG. 4B,reference 200, time t1 to time t2) that is sufficient to ensure fulldischarge of the AC capacitor Xcap. The time period 200 will, forexample, extend for a duration equal to many cycles of the PWM clockand, in particular, may be equal to about one-quarter of the period upto about few periods of the AC power supply voltage Vac. When thecontrol circuit 160 detects that the voltage difference between inputnodes 124 and 134 falls to zero, the signal M1 is deasserted to turn offtransistor switch 122.

Thus, the capacitor Xcap discharges using the selectively actuatedMOSFET of the totem-pole PFC circuit, its power being discharged bydissipation into the resistance of resistor Ricl and possibly theintrinsic resistance of the inductors 104 and 110. Although theresistances are small, they are nonetheless sufficient to dischargecapacitor Xcap. In practice, a current flow of a few milliamperes issufficient to sufficiently rapidly discharge (within from a fewmilliseconds to a few tens of milliseconds) the capacitor Xcap.

Reference is now made to FIG. 5 which schematically shows an embodimentof a power conversion system or circuit 101′ equipped with an ACcapacitor discharge function. Like reference numbers refer to same orsimilar parts. The circuit 101′ differs from the circuit 101 of FIG. 2in that the first and second diodes 150 and 152 have been replaced withthird and fourth thyristors 144 and 146, respectively, in the PFCrectifier circuit 120′. Specifically, the PFC rectifier circuit 120includes a third thyristor 144 (for example, of the cathode-gated type)having a conduction path coupled between the intermediate node 148 andthe first rectified output node 126, and a fourth thyristor 146 (forexample, of the cathode-gated type) having a conduction path coupledbetween the intermediate node 148 and the second rectified output node136. The control circuit 160 further generates the control signals G3and G4 which drive the cathode gates of the third and fourth thyristors144 and 146, respectively.

FIG. 3 shows waveforms for the operation of the PFC rectifier circuit120′ in normal mode. There is no difference in operation in comparisonto the PFC rectifier circuit 120 in FIG. 2.

FIGS. 6A and 6B show waveforms for the operation of the PFC rectifiercircuit 120′ in detecting and responding to a disconnection of an ACinput voltage by discharging an AC capacitor. There is a difference inoperation here as compared to FIGS. 4A-4B and the circuit 101 of FIG. 2.

When the circuit 101′ is disconnected from the AC power supply voltageVac applied to input terminals 100 and 102, the charge on the ACcapacitor Xcap needs to be discharged and the circuit 101′ includes anAC capacitor discharge function using the circuitry of the PFC circuititself to accomplish this goal. The control circuit 160 can sense thedisconnection of the circuit 101 from the AC power supply voltage Vac(or alternatively that there is a DC supply disconnection as discussedabove) by monitoring the voltage at input nodes 124 and 134. If thatsensed voltage at input nodes 124 and 134 fails to zero-cross (i.e., thevoltage difference between input nodes 124 and 134 does not fall tozero), this is indicative of the occurrence of a disconnection of the ACpower supply voltage Vac (or DC supply). In response to sensing thedisconnection event, the control circuit will selectively turn on one ofthe transistor switches 122 or 132 along with simultaneously turning onone of the third and fourth thyristors 144 or 146, depending on polarityof the voltage across AC capacitor Xcap, in order to discharge thestored charge. The control circuit 160 will continue to monitor thevoltage difference between input nodes 124 and 134 and when thatdifference falls to zero the previously turned on transistor switch 122or 132 and previously turned on thyristor 144 or 146 will be turned off

For example, in the event of a disconnection (FIG. 6A, reference 190,time td) where the polarity of the voltage across AC capacitor Xcap ispositive (i.e., powered off under a positive halfwave with theconventions taken in the drawings), the control circuit 160 will detectthrough monitoring of the voltage at input nodes 124 and 134 that nozero-cross occurs (FIG. 6A, reference 192; or alternatively that thereis a DC supply disconnection as discussed above) within a certain timeperiod (FIG. 6A, reference 194, from time td to time t1), wherein thecertain time period may, for example, be in the range of one AC cycle upto about 1-3 seconds. In response thereto, the control circuit 160asserts the signal M2 (FIG. 6A, reference 196, at time t1) to turn onthe second transistor switch 132 and simultaneously asserts the signalG4 (FIG. 6A, reference 202, at time t1) to turn on the fourth thyristor146. Note: the signal G4 could be a pulse of duration sufficiently longenough to reach the latching current of the thyristor, but applying alonger pulse as shown ensures that the thyristor remains on even belowthe holding current so as to ensure full discharge of the capacitor. Thevoltage across the AC capacitor Xcap is discharged (FIG. 6A, reference198) by a capacitor discharge current that flows from node 100 at thepositive terminal of AC capacitor Xcap through inductor 104 to inputnode 124, then through turned on second transistor switch 132 andthrough turned on fourth thyristor 146 to node 148, then throughresistor Rid to node 134, and then through inductor 110 to node 102 atthe negative terminal of AC capacitor Xcap. The control circuit 160simultaneously turns on the second transistor switch 132 throughassertion of signal M2 and the fourth thyristor 146 through assertion ofsignal G4 for a length of time (FIG. 6A, reference 200, time t1 to timet2) that is sufficient to ensure full discharge of the AC capacitorXcap. The time period 200 will, for example, extend for a duration equalto many cycles of the PWM clock and, in particular, may be equal toabout one-quarter of the period up to about few periods of the AC powersupply voltage Vac. When the control circuit 160 detects that thevoltage difference between input nodes 124 and 134 falls to zero, thesignals M2 and G4 are deasserted to turn off transistor switch 132 andthyristor 146.

Conversely, in the event of a disconnection (FIG. 6B, reference 190,time td) where the polarity of the voltage across AC capacitor Xcap isnegative (i.e., powered off under a negative halfwave with theconventions taken in the drawings), the control circuit 160 will detectthrough monitoring of the voltage at input nodes 124 and 134 that nozero-cross occurs (FIG. 6B, reference 192; or alternatively that thereis a DC supply disconnection as discussed above) within a certain timeperiod (FIG. 6B, reference 194, from time td to time t1), wherein thecertain time period may, for example, be in the range of one AC cycle upto about -3 seconds. In response thereto, the control circuit 160asserts the signal M1 (FIG. 6B, reference 196, at time t1) to turn onthe first transistor switch 122 and simultaneously asserts the signal G3(FIG. 6B, reference 202, at time t1) to turn on the third thyristor 144.Note: the signal G3 could be a pulse of duration sufficiently longenough to reach the latching current of the thyristor, but applying alonger pulse as shown ensures that the thyristor remains on even belowthe holding current so as to ensure full discharge of the capacitor. Thevoltage across the AC capacitor Xcap is discharged (FIG. 6B, reference198) by a capacitor discharge current that flows from node 102 at thenegative terminal of AC capacitor Xcap through inductor 110 to inputnode 134, then through resistor Rid to node 148, then through turned onthird thyristor 144 and turned on first transistor switch 122 to node124, and then through inductor 104 to node 100 at the positive terminalof AC capacitor Xcap. The control circuit 160 simultaneously turns onthe first transistor switch 122 through assertion of signal M1 and thethird thyristor 144 through assertion of signal G3 for a length of time(FIG. 6B, reference 200, time t1 to time t2) that is sufficient toensure full discharge of the AC capacitor Xcap. The time period 200will, for example, extend for a duration equal to many cycles of the PWMclock and, in particular, may be equal to about one-quarter of theperiod up to about few periods of the AC power supply voltage Vac. Whenthe control circuit 160 detects that the voltage difference betweeninput nodes 124 and 134 falls to zero, the signals M1 and G3 aredeasserted to turn off transistor switch 122 and thyristor 144.

Thus, the capacitor Xcap discharges using the selectively actuatedMOSFET of the totem-pole PFC circuit, its power being discharged bydissipation into the resistances of resistor Rid, the actuated thyristor144 or 146 and possibly the intrinsic resistance of the inductors 104and 110. Although the resistances are small, they are nonethelesssufficient to discharge capacitor Xcap. In practice, a current flow of afew milliamperes is sufficient to sufficiently rapidly discharge (withinfrom a few milliseconds to a few tens of milliseconds) the capacitorXcap.

Although the first, second, third and fourth thyristors 140, 142, 144,146 are shown in FIG. 5 as being cathode-gates devices, the second andfourth thyristors 142 and 146 could instead be anode-gated devices asshown in FIG. 8.

Reference is now made to FIG. 9 which schematically shows an embodimentof a power conversion system or circuit 301 equipped with an ACcapacitor discharge function. An AC power supply voltage Vac is appliedto input terminals 300 and 302. An AC capacitor Xcap is coupled,preferably connected, to and between the terminals 300 and 302 as partof a filter circuit 303 which also includes an inductor 304 and acapacitor 306 coupled in series between the terminal 300 and a groundnode 308, and an inductor 310 and a capacitor 312 coupled in seriesbetween the terminal 302 and the ground node 308. A PFC rectifiercircuit 320 of the totem-pole type includes a first transistor switch322 (for example, of the n-channel MOSFET type) having a source-drainpath coupled between the input node 324 (at the series connection ofinductor 304 and capacitor 306) and a first rectified output node 326.The PFC rectifier circuit 320 further includes a second transistorswitch 332 (for example, of the n-channel MOSFET type) having asource-drain path coupled between the input node 324 and a secondrectified output node 336. Each of the transistors 322 and 332 includesan intrinsic body diode as shown. An external diode could be added insome cases in reverse parallel of each of the transistors 322 and 332.The PFC rectifier circuit 320 further includes a first thyristor 340(for example, of the cathode-gated type) having a conduction pathcoupled between the input node 334 (at the series connection of inductor310 and capacitor 312) and the first rectified output node 326, and asecond thyristor 342 (for example, of the cathode-gated type) having aconduction path coupled between the input node 334 and the secondrectified output node 336. The PFC rectifier circuit 320 still furtherincludes a third thyristor 344 (for example, of the cathode-gated type)having a conduction path coupled, preferably connected, in series with afirst rush current limiting resistor Ricl1 (for example, of the negativetemperature current (NTC) type) between the input node 334 (at theseries connection of inductor 310 and capacitor 312) and the firstrectified output node 326, and a fourth thyristor 346 (for example, ofthe cathode-gated type) having a conduction path coupled, preferablyconnected, in series with a second rush current limiting resistor Ricl2(for example, of the negative temperature current (NTC) type) betweenthe input node 334 and the second rectified output node 336. Adifferential mode inductor 328 has a first terminal connected to node324 and a second terminal connected to node 330 at the series connectionof transistors 322 and 332. Surge protection diodes (not shown, see FIG.2) may be provided between nodes 324 and 326 and between nodes 334 and336. A DC capacitor Cdc couples, preferably connects, rectified outputnodes 326 and 336 to smooth the rectified voltage and deliver arectified voltage to downstream circuits (for example, a convertercircuit 5 as shown in FIG. 1). The control signals M1 and M2 for drivingthe gates of the first and second transistor switches 322 and 332,respectively, are generated by a control circuit 360. The controlcircuit 360 further generates the control signals G1, G2, G3 and G4which drive the cathode gates of the first, second, third and fourththyristors 340, 342, 344, 346, respectively. The control circuit 360includes inputs for sensing the voltage at input nodes 324 and 334, andfurther includes an input for sensing AC current flow using a currentsensor 362 coupled to sense current flowing through one of the inductors(in this illustrated case, the inductor 310).

FIG. 3 shows waveforms for the operation of the PFC rectifier circuit320 in normal mode. There is no difference in operation in comparison tothe PFC rectifier circuit 120 in FIG. 2 or PFC rectifier circuit 120′ inFIG. 5.

FIGS. 6A and 6B show waveforms for the operation of the PFC rectifiercircuit 320 in detecting and responding to a disconnection of an ACinput voltage by discharging an AC capacitor. The operation here isbasically the same as with the PFC rectifier circuit 120′ in FIG. 5,with the following changes:

a) If a disconnection occurs (detected by the control circuit 360 whenno zero-cross occurs; or alternatively that there is a DC supplydisconnection as discussed above) and the polarity of the voltage acrossAC capacitor Xcap is positive (i.e., powered off under a positivehalfwave with the conventions taken in the drawings), signal M2 isasserted to turn on the second transistor switch 332 and signal G4 issimultaneously asserted to turn on the fourth thyristor 346. The voltageacross the AC capacitor Xcap is discharged by a capacitor dischargecurrent that flows from node 300 at the positive terminal of ACcapacitor Xcap through inductor 304 to input node 324, then throughturned on second transistor switch 332, and then through seriesconnected resistor Ricl2 and turned on fourth thyristor 346 to node 334,and then through inductor 310 to node 302 at the negative terminal of ACcapacitor Xcap; and

b) if a disconnection occurs (detected by the control circuit 360 whenno zero-cross occurs; or alternatively that there is a DC supplydisconnection as discussed above) and the polarity of the voltage acrossAC capacitor Xcap is negative (i.e., powered off under a negativehalfwave with the conventions taken in the drawings), signal M1 isasserted to turn on the first transistor switch 322 and signal G3 issimultaneously asserted to turn on the third thyristor 344. The voltageacross the AC capacitor Xcap is discharged by a capacitor dischargecurrent that flows from node 302 at the negative terminal of ACcapacitor Xcap through inductor 310 to input node 334, then throughseries connected resistor Ricl1 and turned on third thyristor 344, andthen through turned on first transistor switch 322 to node 324, and thenthrough inductor 304 to node 300 at the positive terminal of ACcapacitor Xcap.

Thus, the capacitor Xcap discharges using the selectively actuatedMOSFET of the totem-pole PFC circuit, its power being discharged bydissipation into the resistances of resistor Ricl1 and actuatedthyristor 344 (or resistor Ricl2 and actuated thyristor 446) andpossibly the intrinsic resistance of the inductors 304 and 310. Althoughthe resistances are small, they are nonetheless sufficient to dischargecapacitor Xcap. In practice, a current flow of a few milliamperes issufficient to sufficiently rapidly discharge (within from a fewmilliseconds to a few tens of milliseconds) the capacitor Xcap.

Although the first, second, third and fourth thyristors 340, 342, 344,346 are shown in FIG. 9 as being cathode-gates devices, the second andfourth thyristors 342 and 346 could instead be anode-gated devices (see,as shown in FIG. 8 with thyristors 142 and 146).

The illustration of the voltage across the capacitor Vcap as shown inFIGS. 4A, 4B, 6A and 6B at the time of the disconnection is asimplification that ignores any current drawn due to continued operationof downstream circuits such as the switching converter 5 (FIG. 1). Withthe presence of such a connected load, there will be an abrupt decrease(faster than the sinusoidal decrease of voltage Vac) of voltage acrossthe capacitor Xcap at the time td and shortly thereafter until thevoltage levels out (reference 192). The subsequent failure to have azero-cross (for example, within a time period of one or two cycles of ACvoltage following the disconnection event; or alternatively that thereis a DC supply disconnection as discussed above) is detected by thecontrol circuit 160, 360 and this will trigger performance of thecapacitor discharge function.

An advantage of the described embodiments is that their implementationonly requires the generation of a specific control of the switchingtransistors (and perhaps also the thyristors) of a totem-pole PFCcircuit in order to provide the AC capacitor discharge function.

An advantage of the described embodiments is that the discharge of theAC capacitor Xcap is particularly simple and uses circuits of lowcomplexity.

Various embodiments and variations have been described. Those skilled inthe art will understand that certain features of these variousembodiments and variations may be combined, and other variations willoccur to those skilled in the art.

Finally, the practical implementation of the described embodiments andvariations is within the abilities of those skilled in the art based onthe functional indications given hereabove. In particular, the selectionof the time period for which voltage Vac has disappeared before thedischarge of capacitor Xcap may vary, provided that it is compatiblewith the maximum time period required to discharge capacitor Xcap, whichis generally set by standards. A time period from a few tens ofmilliseconds to a few seconds is a preferred choice, more preferably inthe order of from 40 ms to 2 s.

1. A circuit, comprising: a first capacitor having first and secondelectrodes respectively coupled to first and second power supply inputnodes; a first switching transistor having a conduction path coupledbetween the first power supply input node and a first DC node; a secondswitching transistor having a conduction path coupled between the firstpower supply input node and a second DC node; a first thyristor having aconduction path coupled between the second power supply input node andthe first DC node; a second thyristor having a conduction path coupledbetween the second power supply input node and the second DC node; aresistor coupled between the second power supply input node and anintermediate node; a third thyristor having a conduction path coupledbetween the intermediate node and the first DC node; a fourth thyristorhaving a conduction path coupled between the intermediate node and thesecond DC node; and a control circuit configured to sense adisconnection of input power to the first and second power supply inputnodes and in response thereto turn on one of the first and secondswitching transistors and one of the third and fourth thyristors todischarge the first capacitor through the resistor.
 2. The circuit ofclaim 1, wherein sensing disconnection comprises sensing by the controlcircuit of a certain condition with respect to a voltage between thefirst and second power supply input nodes within a time period.
 3. Thecircuit of claim 1, wherein sensing disconnection comprises delayingturn on of one of the first and second switching transistorsrepetitively for a time period.
 4. The circuit of claim 1, wherein thefirst, second, third and fourth thyristors are cathode-gate devices. 5.The circuit of claim 1, wherein the one of the first and secondthyristors is a cathode-gate device and another of the first and secondthyristors is an anode-gate device.
 6. The circuit of claim 1, whereinthe one of the third and fourth thyristors is a cathode-gate device andanother of the third and fourth thyristors is an anode-gate device. 7.The circuit of claim 1, wherein the first and second switchingtransistors are MOSFETs.
 8. The circuit of claim 1, wherein the controlcircuit is further configured to selectively turn on one of the firstand second thyristors in a normal operational mode for producing arectified DC voltage across the first and second DC nodes from inputpower at the first and second power supply input nodes.
 9. The circuitof claim 8, wherein the control circuit is further configured to apply apulsed control signal to one the first and second switching transistorsin the normal operational mode while said one of the first and secondthyristors is selectively turned on.
 10. The circuit of claim 9, whereinthe pulsed control signal is pulse width modulated.
 11. The circuit ofclaim 1, wherein said one of the first and second switching transistorsand said one of the third and fourth thyristors which are selectivelyturned on to discharge the first capacitor is dependent on a sign of avoltage for the input power.
 12. The circuit of claim 1, wherein thecontrol circuit senses the disconnection of the input power if an ACvoltage between the first and second power supply input nodes for theinput power fails to zero-cross within a time period.
 13. The circuit ofclaim 12, wherein the time period is a range from one period of the ACvoltage to a few periods of the AC voltage.
 14. The circuit of claim 1,wherein the control circuit senses the disconnection of the input powerif a voltage between the first and second power supply input nodesdecreases within a time period when one of the first and secondswitching transistors is turned on.
 15. The circuit of claim 1, whereinthe control circuit senses the disconnection of the input power if asensed current does not increase to a certain degree within a timeperiod when one of the first and second switching transistors is turnedon.
 16. The circuit of claim 1, further comprising an inductor coupledbetween the first power supply input node and a node at a seriescoupling of the first and second switching transistors.
 17. A circuit,comprising: a first capacitor having first and second electrodesrespectively coupled to first and second power supply input nodes; afirst switching transistor having a conduction path coupled between thefirst power supply input node and a first DC node; a second switchingtransistor having a conduction path coupled between the first powersupply input node and a second DC node; a first thyristor having aconduction path coupled between the second power supply input node andthe first DC node; a second thyristor having a conduction path coupledbetween the second power supply input node and the second DC node; afirst resistor and a third thyristor having a conduction path coupled inseries between the second power supply input node and the first DC node;a second resistor and a fourth thyristor having a conduction pathcoupled in series between the second power supply input node and thesecond DC node; and a control circuit configured to sense adisconnection of input power to the first and second power supply inputnodes and in response thereto turn on one of the first and secondswitching transistors and one of the third and fourth thyristors todischarge the first capacitor through one of the first and secondresistors coupled in series with said one of the third and fourththyristors.
 18. The circuit of claim 17, wherein sensing disconnectioncomprises sensing by the control circuit of a certain condition withrespect to a voltage between the first and second power supply inputnodes within a time period.
 19. The circuit of claim 17, wherein sensingdisconnection comprises delaying turn on of one of the first and secondswitching transistors repetitively for a time period.
 20. The circuit ofclaim 17, wherein the first, second, third and fourth thyristors arecathode-gate devices.
 21. The circuit of claim 17, wherein the one ofthe first and second thyristors is a cathode-gate device and another ofthe first and second thyristors is an anode-gate device.
 22. The circuitof claim 17, wherein the one of the third and fourth thyristors is acathode-gate device and another of the third and fourth thyristors is ananode-gate device.
 23. The circuit of claim 17, wherein the first andsecond switching transistors are MOSFETs.
 24. The circuit of claim 17,wherein the control circuit is further configured to selectively turn onone of the first and second thyristors in a normal operational mode forproducing a rectified DC voltage across the first and second DC nodesfrom input power at the first and second power supply input nodes. 25.The circuit of claim 24, wherein the control circuit is furtherconfigured to apply a pulsed control signal to one the first and secondswitching transistors in the normal operational mode while said one ofthe first and second thyristors is selectively turned on.
 26. Thecircuit of claim 25, wherein the pulsed control signal is pulse widthmodulated.
 27. The circuit of claim 17, wherein said one of the firstand second switching transistors and said one of the third and fourththyristors which are selectively turned on to discharge the firstcapacitor is dependent on a sign of a signal for the input power. 28.The circuit of claim 17, wherein the control circuit senses thedisconnection of the input power if an AC voltage between the first andsecond power supply input nodes for the input power fails to zero-crosswithin a time period.
 29. The circuit of claim 28, wherein the timeperiod is a range from one period of the AC voltage to a few periods ofthe AC voltage.
 30. The circuit of claim 17, wherein the control circuitsenses the disconnection of the input power if a voltage between thefirst and second power supply input nodes decreases within a time periodwhen one of the first and second switching transistors is turned on. 31.The circuit of claim 17, wherein the control circuit senses thedisconnection of the input power if a sensed current does not increaseto a certain degree within a time period when one of the first andsecond switching transistors is turned on.
 32. The circuit of claim 17,further comprising an inductor coupled between the first power supplyinput node and a node at a series coupling of the first and secondswitching transistors.